Fixation system for an implantable medical device

ABSTRACT

A medical implant, such as an implantable component ( 22 ) of a tissue-stimulating prosthesis. One example of such a prosthesis being a cochlear implant. The component ( 22 ) is adapted to be implanted at or adjacent a tissue surface within the recipient, such as a bone surface. The component ( 22 ) has a housing and at least one flange ( 42 ) extending outwardly therefrom. The flange ( 42 ) can be secured to the tissue surface via a tissue fixation device, such as a bone screw ( 43 ).

FIELD OF THE INVENTION

The present invention resides in an improved method of mounting animplantable component of an implantable medial device, such as acochlear implant package, securely in the head region of a recipient.

BACKGROUND OF THE INVENTION

In many people who are profoundly deaf, the reason for deafness isabsence of, or destruction of, the hair cells in the cochlea whichtransduce acoustic signals into nerve impulses. These people are unableto derive suitable benefit from conventional hearing aid systems, nomatter how loud the acoustic stimulus is made, because there is damageto or absence of the mechanism for nerve impulses to be generated fromsound in the normal manner.

It is for this purpose that cochlear implant systems have beendeveloped. Such systems bypass the hair cells in the cochlea anddirectly deliver electrical stimulation to the auditory nerve fibres,thereby allowing the brain to perceive a hearing sensation resemblingthe natural hearing sensation normally delivered to the auditory nerve.

Cochlear implant systems have typically consisted of essentially twocomponents, an external component commonly referred to as a processorunit and an internal implanted component commonly referred to as areceiver/stimulator unit. Traditionally, both of these components havecooperated together to provide the sound sensation to a user.

The external component has traditionally consisted of a microphone fordetecting sounds, such as speech and environmental sounds, a speechprocessor that converts the detected sounds, particularly speech, into acoded signal, a power source such as a battery, and an externaltransmitter antenna coil.

The coded signal output by the speech processor is transmittedtranscutaneously to the implanted receiver/stimulator unit situatedwithin a recess of the temporal bone of the user. This transcutaneoustransmission occurs via the external transmitter antenna coil which ispositioned to communicate with an implanted receiver antenna coilprovided with the receiver/stimulator unit.

This communication serves two essential purposes, firstly totranscutaneously transmit the coded sound signal and secondly to providepower to the implanted receiver/stimulator unit. Conventionally, thislink has been in the form of a radio frequency (RF) link, but other suchlinks have been proposed and implemented with varying degrees ofsuccess.

The implanted receiver/stimulator unit traditionally includes a receiverantenna coil that receives the coded signal and power from the externalprocessor component, and a stimulator that processes the coded signaland outputs a stimulation signal to an intracochlear electrode assemblywhich applies the electrical stimulation directly to the auditory nerveproducing a hearing sensation corresponding to the original detectedsound.

Traditional implanted receiver/stimulator units are positioned withinthe head of the recipient by drilling a bed into and through theposterior section of the mastoid bone lying behind the recipient's ear.Such a bed is usually made by drilling the bone down to the lining ofthe brain or dura mater, so that the receiver/stimulator unit ismaintained in position and does not protrude excessively past the skullsurface.

The receiver/stimulator unit manufactured by the present Applicant has apackage made from titanium which houses the stimulation electronics andwhich is fitted into a bed created in the mastoid bone. A receiverantenna coil extends from the rear end of the package and liessuperficial to the bone. Other cochlear implants have included packagesmade from a ceramic material which are usually placed completely withinthe bed drilled down to the dura mater.

Various techniques have been implemented in order to mount or fix thedevice in place and to ensure that the device does not undergo movementonce implanted.

One such technique has been to drill holes in the bone surrounding thedevice and to use sutures or Dacron ties to hold the device in place.One problem with this approach is that drilling of the holes into thesurrounding bone can be a difficult and time consuming procedure, andespecially for young children, much care must be taken by the surgeon toensure that the drilling does not perforate the dura mater, as the skullthickness in such cases can be quite thin. Further to this, the sutureor Dacron ties may not be sufficiently strong enough to withstand asubstantial impact to a region of the head adjacent the device and as aresult, such a force may dislodge the device from its desired position.In addition, it has been found that if a suture or Dacron tie isinadvertently placed across an inappropriate section of the device, suchas across a strain relief of the electrode lead, the suture/tie maycause the lead/device to undergo fatigue and cause failure at thislocation.

Another technique used to secure the implant device in place is for thesurgeon to craft a suitable well or bed in the cranial bone that iscapable of maintaining the device in place without the need of suturesor ties. Such a technique relies upon the shape of the well or bed beingsuch that the surrounding bone can hold the device in place. Thistechnique is not always possible depending upon the thickness of thesurrounding bone and the age and anatomy of the recipient.

Therefore, there is a need to provide a fixation method for animplantable hearing prosthesis that is capable of securely maintainingthe device in place in a desired region of the recipient's head withoutthe need for additional sutures or ties.

Any discussion of documents, acts, materials, devices, articles or thelike which has been included in the present specification is solely forthe purpose of providing a context for the present invention. It is notto be taken as an admission that any or all of these matters form partof the prior art base or were common general knowledge in the fieldrelevant to the present invention as it existed before the priority dateof each claim of this application.

SUMMARY OF THE INVENTION

Throughout this specification the word “comprise”, or variations such as“comprises” or “comprising”, will be understood to imply the inclusionof a stated element, integer or step, or group of elements, integers orsteps, but not the exclusion of any other element, integer or step, orgroup of elements, integers or steps.

According to a first aspect, the present invention is a medical implantfor implantation within a body of a recipient, the medical implantcomprising a hermetically sealed housing, said housing having at leastone flange extending outwardly therefrom that is securable to tissuewithin the body of the recipient.

In this aspect, the flange can be securable to a suitable tissuesurface. While a bone surface is preferred, it can be envisaged that theflange could be securable to a other suitable tissues within the body,including cartilage and tendons.

In one embodiment, the implant can be an implantable component of atissue-stimulating prosthesis. In a preferred embodiment, thetissue-stimulating prosthesis is a cochlear implant. The implantablecomponent of the cochlear implant preferably comprises areceiver/stimulator package of such an implant. While the presentapplication will hereinafter refer to cochlear implants, it is to beunderstood that the invention has a potential wider application to otherimplantable tissue-stimulating prostheses.

In one embodiment, the housing of the implantable component can beadapted to be placed on the surface of the bone of the recipient. In thecase of a cochlear implant, this bone would likely be the mastoid bone.In another embodiment, a bed or well can be formed in the surface of thebone, such as the mastoid bone, such that the housing can be positionedin the well or bed.

In one embodiment, a flange can extend outwardly from the housing in atleast one direction. More preferably, the housing has at least twoflanges extending outwardly therefrom. In this embodiment, the flangespreferably extend in substantially opposite directions relative to eachother. In a further embodiment where there are two flanges, therespective flanges can extend in opposite directions relative to eachother.

In one embodiment, at least one flange preferably extends from a firstor upper surface of the housing. The first surface of the housing ispreferably the outer surface of the housing on implantation of thecomponent.

In another embodiment, at least one flange can extend from a second orlower surface of the housing. In this embodiment, the second surfacepreferably faces inwardly, such that the surface normally abuts with oris embedded in the bone of the recipient receiving the implantablecomponent.

In a still further embodiment, at least one flange can extend from thehousing at a location between the first and second surfaces of thehousing. In one embodiment, the flange can extend outwardly from alocation that is approximately midway between the first and secondsurfaces.

In yet another embodiment and where there is more than one flange, oneof such flanges may extend from the first or upper surface while anotherextends from the second or lower surface. Other flange combinations canbe envisaged.

In a further embodiment, the flanges extending from the housing comprisepart of a plate mounted to the housing of the implantable component. Inone embodiment, the plate can be removably or non-removably mounted tothe housing.

Each of the flanges are preferably adapted to abut the tissue surface ofthe recipient following implantation of the implantable component. Inone embodiment, the flanges are preferably conformable to the tissuesurface. In this regard, the flanges can be formed from a malleablematerial that allows the flanges to be conformed to the surface of thetissue, such as bone. In another embodiment, the flanges can beconstructed so as to be conformable to the tissue surface. In thisregard, the flanges may have a thickness that allows the flanges to besuitably conformable during the surgical procedure. In a still furtherembodiment, both the properties of the material and the construction ofthe flanges may play a role in ensuring the flanges are malleable andconformable to the tissue surface. The flanges are preferablyconformable by finger pressure exertable on the flanges by a surgeonduring the surgical implant of the implantable component.

The degree of conformation of the flanges necessary to ensure theflanges conform to the tissue surface will depend on the position of theflanges and/or whether the housing of the component is embedded withinthe tissue, such as within a well or bed within the bone. Where theflanges extend from the first surface of the housing, the downward angleof the flanges necessary so as to abut with the tissue surface willdepend on the degree to which the housing is embedded within the tissue.The downward angle of the flange is likely to be less when the housingis at least partially embedded in the tissue in comparison to thesituation where the housing essentially is sitting on the tissuesurface.

In one embodiment, the flanges can have a thickness between about 0.1 mmand 0.3 mm. The flanges can be formed from a malleable material.

In one embodiment, the flanges can constitute an integral extension ofthe housing of the implantable component. In another embodiment, theflanges can be formed separately and mounted to the housing. Techniquessuch as welding and brazing can be envisaged as techniques for mountingthe flanges to the housing of the implantable component. In anotherembodiment, one or more flanges may be an integral extension of thehousing while one or more may be formed separately and mounted to thehousing.

In another embodiment, the flanges can be removably mounted to thehousing. In this embodiment, the flanges or housing of the implantablecomponent can be provided with engagement means adapted to engage withthe housing or flanges, respectively. In one embodiment, the housing canhave one or more clips adapted to engage with the flanges. In thisembodiment, it is envisaged that the flanges may not be mounted to thehousing until surgery is underway and the size and shape of the flangesrequired for that particular surgery have been determined. Stillfurther, removably mounted flanges provide the surgeon with the optionof not using the flanges at all.

In one embodiment, the flanges can be formed of titanium, such asmalleable titanium. Tn this and other embodiments, the housing of theimplantable component can also be formed from titanium. In anotherembodiment, the housing of the implantable component and/or the flangescan be formed of other materials, including suitable biocompatiblemetallic, ceramic and polymeric materials. In this regard, the flangesand housing do not need to be formed of the same material. For example,the flanges could be formed of a polymeric material, such aspolypropylene or polytetrafluoroethylene, while the housing is formed ofa ceramics or metallic material.

As defined above, the flanges are preferably securable to the surface ofthe tissue within the recipient. In one embodiment, one or more of theflanges can have orifices passing therethrough. These orifices can beadapted to receive tissue fixation devices, such as screws, clips and/ornails, including bone screws, bone clips and bone nails. In oneembodiment, the screws can be countersunk, or have a round head. Stillfurther, the tissue fixation devices can be resorbable.

In one embodiment, the housing is preferably adapted to be secured tothe tissue surface at the site of each flange. It will, however, beappreciated that there may be instances where it is not possible to usea particular flange due to a previous cavity having been formed in theselected tissue, or, for example, the presence of a skull growth line,or a region of bone weakness.

In the above embodiments, the flanges and/or housing can be coated witha layer of silicone rubber or other suitable elastomeric material. Thetissue fixation devices would preferably be accessible by means of aslit or hole formed or formable in the coating material.

According to a second aspect, the present invention is a medical implantfor implantation within a body of a recipient, the medical implantcomprising a hermetically sealed housing, said housing having at leastone flange extending outwardly therefrom that is securable andconformable to tissue within the body of the recipient.

In this aspect, the said at least one flange can be formed of amalleable material. In this aspect, the implant, housing and flange canhave any one or more of the features defined herein with reference tothe first aspect.

In the case of a cochlear implant, an electrically conducting leadpreferably extends from the receiver/stimulator package to an electrodearray. The lead preferably exits the package such that it is extendableinto the cochlea of the recipient on appropriate positioning of theimplantable component within the recipient. In a preferred embodiment,the lead preferably extends from the implanted package to the cochleavia a posterior tympanotomy positioned at the bottom of a mastoidcavity. Other lead positions and geometries are can, however, beenvisaged.

The present invention provides a housing of an implantable componenthaving one or more flanges for use in securing the component to a tissuesurface of the recipient. In addition to supporting the component, theflanges have the additional characteristic of serving to protect thecomponent from inadvertent dislodgment following an impact that mightotherwise dislodge the component if positioned and mounted usingconventional techniques.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example only, a preferred embodiment of the invention is nowdescribed with reference to the accompanying drawings, in which:

FIG. 1 is a pictorial representation of a conventional cochlear implantsystem;

FIG. 2 is a representation of a conventional receiver/stimulator unitpositioned in a bed fashioned in the mastoid bone according toconventional surgical techniques;

FIG. 3 is a representation of a typical prior art method of fixing thereceiver/stimulator unit in place during surgery;

FIG. 4 is a plan view of one embodiment of the present invention;

FIG. 5a is an end view of the embodiment shown in FIG. 4 mounted to abone surface of a skull;

FIG. 5b is a side view of the embodiment shown in FIG. 4 mounted to abone surface of a skull;

FIGS. 6a and 6b are an end view and a side view, respectively, of theembodiment shown in FIG. 4 with the implant being sunk lower into theskull than that shown in FIG. 5 a;

FIGS. 7a and 7b are an end view and a side view, respectively, of analternative embodiment of the present invention;

FIGS. 7c and 7d are an end view and a side view, respectively, of yetanother alternative embodiment of the present invention;

FIGS. 8a and 8b are an end view and a side view, respectively, of yetanother embodiment of the present invention;

FIGS. 9a and 9b are an end view and a side view, respectively, of analternative embodiment of the present invention;

FIG. 10 is an alternative embodiment of the present invention havingdetachable flange portions;

FIGS. 11a and 11b are an end view and a side view, respectively, of theembodiment shown in FIG. 10;

FIGS. 12a and 12b are an end view and a side view, respectively, of theembodiment shown in FIG. 10 where the implant has been sunk lower intothe skull than that depicted in FIGS. 11a and 11 b;

FIG. 13 is yet another embodiment of the present invention having adetachable flange plate; and

FIGS. 14a and 14b are an end view and a side view, respectively, of theembodiment shown in FIG. 13.

PREFERRED MODE OF CARRYING OUT THE INVENTION

Before describing the features of the present invention, it isappropriate to briefly describe the construction of one type of knowncochlear implant system with reference to FIG. 1.

Known cochlear implants typically consist of two main components, anexternal component including a speech processor 29, and an internalcomponent including an implanted receiver and stimulator unit 22. Theexternal component includes a microphone 27. The speech processor 29 is,in this illustration, constructed and arranged so that it can fit behindthe outer ear 11 and is held in place behind the outer ear 11 via anear-hook arrangement. Alternative versions may be worn on the body.Attached to the speech processor 29 via a cable 13 is a transmitterantenna coil 24 that transmits electrical signals to the implanted unit22 via a radio frequency (RF) link.

The implanted component includes a receiver antenna coil 23 forreceiving power and data from the transmitter coil 24. A cable 21extends from the implanted receiver and stimulator unit 22 to thecochlea 12 and terminates in an electrode array 20. The signals thusreceived are applied by the array 20 to the basilar membrane 8 and thenerve cells within the cochlea 12 thereby stimulating the auditory nerve9. The operation of such a device is described, for example, in U.S.Pat. No. 4,532,930.

FIG. 2 shows in more detail the surgical placement of the implantedreceiver and stimulator package 22 of FIG. 1, according to conventionalpractices. The package 22 is in the form of a capsule, for example atitanium capsule, which houses the necessary circuitry required for theimplant to operate as desired. The receiver antenna coil 23 is shownencapsulated in a material, such as silicone rubber, to provide aprotective body and ensure fatigue resilience. A magnet 30 is shownpositioned within the coil to assist in the alignment of the transmittercoil 24 with the receiver antenna coil 23 as discussed previously. As isshown, a bed is drilled into the bone 31 to maintain the package 22 inposition. This bed is typically round or ovoid to match the shape of thepackage. The bed is typically made in the mastoid bone and mastoid angleof the parietal bone in the region of the asterion. Typically, the bedis fashioned initially with a cutting burr, and then completed with adiamond paste burr and a template is typically used to ensure that thebed is fashioned to the correct size. As is shown, the bed may bedrilled down to the lining of the brain, or dura mater 32, particularlyfor young children with thin skulls, and it is for this reason that adiamond paste burr may be used when approaching the dura and when thedura is exposed, to minimise the risk of tearing of the dura 32.

As can be appreciated from FIG. 2, any impact in the direction shown bythe arrow A of FIG. 2, has the potential for the package to tear thedura 32 and enter the cranial cavity, potentially causing damage to thesensitive structures of the brain. Further to this, an impact to thehead region of the recipient, particularly in the direction shown byarrow B, has the potential to dislodge the implant from its bed withinthe skull bone. Such dislodgment can cause damage to the area of thehead adjacent the device as well as discomfort to the recipient. Anydislodgment of the device also has the potential to require furthersurgical procedures to relocate the device in the desired positionwithin the head of the recipient. Further to this, such dislodgment ofthe device may cause the location of the strategically positionedelectrodes of the electrode array to be altered, requiring possiblere-implantation to rectify this problem.

FIG. 3 depicts a typical fixation technique used to secure the device 22in place prior to wound closure during a surgical procedure. In such atechnique, prior to fixation of the device 22, small tunnels 35 aredrilled in the bone on either side of the package bed to place ties orsutures 36 to hold the receiver/stimulator package in place. Aspreviously mentioned, in an infant, such a procedure is not recommended,as the bone is thin and the drill may abrade the dura. For children, onemethod of securing the device in place is to tie the device down withligatures placed through the temporalis and deep fascia, and to alsostitch the anteriorly based facial flap over the package.

One embodiment of the fixation system according to the present inventionis shown in FIGS. 4, 5 a and 5 b. In this embodiment, the implantpackage is once again shown by numeral 22, which preferably consists ofa titanium casing enclosing the implant electronics. The silicone rubbermaterial encapsulating the receiver coil is shown as 41, which inpractice would encapsulate both the receiver antenna coil and apositioning magnet (not shown) as is known in the art. Two malleableflanges 42 are shown extending from the implant package 22. Thesemalleable flanges 42 are suitably sized and shaped to be integrated withthe implant package 22 so that the package 22 can be secured in place bysecuring the flanges to the skull via skull attachment devices 43 (ie.bone screws or pins). While depicted being securable to a bone surface,it will be appreciated that the present invention has application formounting medical implants to other bodily tissues.

The malleable flanges 42 are preferably made from a titanium materialand, in the depicted embodiment, are attached to the titanium implantpackage 22 by welding. Alternatively, the flanges 42 may be madeintegral with the implant package 22, and may merely be extension of thepackage 22. It is envisaged that other metals may be used for theimplant package 22 and flanges 42, for example, any biocompatible metalsuch as stainless steel. It would, however, be preferable that thematerial used for the implant package 22 and/or flanges 42 benon-magnetic to allow MRI compatibility.

The skull attachment devices 43 are typically surgical screws andpreferably have a low profile so they do not cause tissue erosion in theregion of the head surrounding the implant, or produce a noticeableprotuberance. Preferably, the flanges 42 and skull attachment devices 43are coated in a silicone rubber to prevent tissue erosion, with theskull attachment devices 43 being accessed by means of a slit or hole inthe silicone rubber above the skull attachment devices 43.

As shown in FIGS. 5a and 5 b, the flanges 42 are attached to theuppermost surface of the implant package 22, with part of the implantpackage 22, hereby referred to as the implant pedestal 44, being sunkinto the skull. In these figures, the dotted line represents the line ofthe skull. It should be appreciated that in all embodiments shown thepedestal 44 is not essential to the invention, and is only shown here toillustrate that the present invention is applicable to implants ofvariable configurations. It should also be appreciated that other suchembodiments are possible and it is not necessary for the flanges 43 tobe attached to the uppermost surface of the package 22, as the flanges43 could be equally connected to other regions of the implant package22.

As can be appreciated in FIGS. 5a and 5 b, the flanges 43 must besufficiently robust and yet sufficiently malleable to enable them to beformed and manipulated to fit the shape of the recipient's skull. Asthis manipulation typically occurs during the surgical implantationprocedure, it is important that the forces required to manipulate theflanges 43 can be performed by the surgeon using finger pressure only.As the anatomy in this area of the head varies from patient to patient,it is desirable to form the flanges 43 so as to have a flush fit againstthe skull so as to maintain a low profile of the device and to reducethe occurrence of tissue erosion. In this regard, the flanges 43 wouldpreferably only need to be manipulated to a relatively small degree.

As surgical methods and preferences vary from surgeon to surgeon, it isimportant that the present invention can also be adapted to meet suchvariations. As can be seen in FIGS. 6a and 6 b, some surgeons may preferto partially sink the implant package 22 into the skull bone, with thedotted line in both figures representing the line of the skull. In sucha case, the flanges 42 must be malleable to allow the surgeon to bendthem to the correct position to suit the preferred depth of the implantbed or well. By a direct comparison between the implants depicted inFIGS. 5a and 5b and FIGS. 6a and 6 b, it can be seen that in the casewhere the implant package is sunk to a deeper depth (FIGS. 6a and 6b ),the angle (Φ), the downward angle with which the flanges 42 exit theimplant package 22, can be adjusted accordingly. The angle (Φ) shown inFIGS. 6a and 6b has been increased to compensate for the greater depthof the implant bed than that shown in FIGS. 5a and 5 b. Thismalleability characteristic can be achieved by selection of the materialand/or thickness and/or geometry of the flanges 42. For example,annealed titanium of a thickness between 0.1-0.3 mm of the flange shapeshown in FIG. 4, would be sufficiently malleable for a surgeon to bendduring surgery.

FIGS. 7a and 7b show an alternative embodiment of the present invention.In this embodiment, the malleable flanges 42 are arranged to exit theimplant package 22 from a lower surface of the package, but with theimplant pedestal 44 still adapted to be positioned within a well formedin the skull. In this embodiment, the flanges 42 would preferably lieflush with the skull, and such a design may be more preferable foryounger children having thinner skin flaps over the implant.

FIGS. 7c and 7d show a further variation of the embodiment of FIGS. 7aand 7 b. In this embodiment, the malleable flanges 42 are also arrangedto exit the implant package 22 from the lower surface of the implantpackage. In this embodiment, there is no need to drill into therecipient's skull to form a bed for a pedestal 44 and the implantpackage can be quickly and securely fixed to the recipient's skull viaskull attachment devices 43. In this embodiment, the implant package 22is also provided with a domed profile to avoid the occurrence of tissueerosion.

FIGS. 8a and 8b show yet another variation of the present invention. Inthis embodiment, the flanges 42 are attached to the uppermost surface ofthe implant package 22, with the implant package 22 positioned below theflanges 42. In this embodiment, a thin implant package can be sunk intothe skull, ensuring that there is a minimal protuberance caused by thedevice.

FIGS. 9a and 9b depict yet another embodiment of the present inventionwhere the flanges 42 are attached at about the mid-line of the implantpackage 22, with the implant package 22 being typically partly buried inthe skull. In this embodiment, the flange 42 would also preferably bepositioned flush with the skull, while the bed or well for the implantpackage would be fashioned in such a manner so as the antenna body isalso partly sunk into the skull as shown. Such an embodiment does notrequire a pedestal 44 and ultimately provides a low profile implant thatdoes not produce an unsightly protuberance and the problems associatedtherewith.

It should be appreciated that each of the flanges 42 shown in the abovementioned embodiments could be made from a plastic or elastomericmaterials bonded to the implant package 22. For example, the siliconerubber coating of the implant package 22 can be extended to create asilicone rubber flange which may be secured to the skull via appropriatemeans. Further, a plastic material, such as PTFE or polyurethane, can beembedded within the silicone rubber coating of the implant package 22 toform a flange. Such a device may also be attached to the implant packagevia a mechanical interlock. It may also be possible to make the flangeof a composite or combination of materials. For example, a Dacron meshmay be used as a reinforcing structure to strengthen the silicone rubbercoating. PTFE, polyurethane or carbon fibre materials may also be usedas a reinforcing member.

By providing the flange made from a plastic or elastomeric material, itthen becomes possible for the surgeon to remove or cut-off the flangeduring the surgical procedure should they not wish to use such afixation method. This results in the fixation mechanism being anoptional feature. Such a flange would also be easier to form and alterthe shape thereof to more appropriately conform to the shape of therecipient's skull. Further, a flange made from a plastic or elastomericmaterial is softer than a metallic flange and will therefore be lessprone to causing tissue erosion.

FIGS. 10, 11 a, 11 b, 12 a, and 12 b depict yet another embodiment ofthe present invention. In this embodiment, the flanges 42 are detachablefrom the implant package 22. Each flange 42 is attached to the implantpackage 22 via lugs or clips 50 protruding from the side of the case ofthe implant package 22. The flanges 42 are received in the lugs or clips50 and can be securely attached to the skull via appropriate skullattachment devices 43. This embodiment has the advantage that there isno need for a separate flange plate which adds thickness to the implantpackage. Further, the use of the flanges is optional, should the surgeonprefer not to use the flanges to secure the implant in place.

As is shown in FIGS. 11 a, 11 b, 12 a and 12 b, this embodiment allowsthe implant to be sunk into the skull to varying depths as decided bythe surgeon during surgery. FIGS. 11a and 11b depict the implantpositioned to a depth where only the pedestal 44 is sunk into the skull,with the detachable flanges 42 fixing the device in place throughengagement with the lugs or clips 50 formed on the implant package 22.FIGS. 12a and 12b depict the implant being positioned deeper within theskull bone, with the detachable flanges 42 engaging with the lugs orclips 50 to secure the device in place.

FIG. 13 depicts yet another embodiment of the present invention. In thisembodiment, a detachable flange plate 55 is used instead of separateflange elements that are integrally fixed or detachably fixed as shownin previous embodiments. The detachable flange plate 55 extends over andacross the implant package 22, with skull attachment devices 43 beingused to secure the plate 55 and therefore the implant package 22 inposition. As is shown in FIG. 13, the plate 55 could include a cut-outsection 56 should an electrode be positioned on the implant packagecasing 22 (as is the case with some currently existing implants) suchthat body fluids can access the electrode. There may also be a need toprovide washers 57 to assist in securing the plate to the skull via theskull attachment devices, especially if a non-metallic plate is used.The plate 55 can be malleable to conform with the desired shape of therecipient's skull, and the shape can easily be manipulated by thesurgeon during surgery.

FIGS. 14a and 14b depicts the plate in use. As can be appreciated, theshape of the plate can be easily manipulated to cater for variabledepths of implantation of the package 22 into the skull.

The embodiment as shown in FIG. 13 has advantages in that the plate 55is an optional feature, and should a surgeon decide that such a methodof fixation is not suitable for the patient during surgery, then thesurgeon can choose not to use the plate 55. Further to this, the plate55 could be used with existing implants without any need to modify thedesign of present implant packages to accommodate such a securingmechanism. This is important, particularly in the case of implantsutilising a ceramic or non-metallic casing, as these implants do notallow for metal flanges to be welded to the non-metallic casing.

As alluded to above, the plate 55 may be made of a non-metallicmaterial, such as a biocompatible plastic since there is no need forwelding of the plate to the implant package 22. Such a plate wouldovercome the need to provide a coating of silicone rubber to the surfaceof the plate to soften it and prevent tissue erosion. In this regard,the plate 55 could be made of a polyurethane or PTFE which are strong,relatively inelastic materials suited to this application. However, itshould be envisaged that other plastics may also be used which exhibitthe desired properties.

It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the invention as shown inthe specific embodiments without departing from the spirit or scope ofthe invention as broadly described. The present embodiments are,therefore, to be considered in all respects as illustrative and notrestrictive.

What is claimed is:
 1. A medical implant configured to be attached to askull bone of a recipient, the implant comprising: a hermetically sealedhousing encasing an electrical apparatus utilized to evoke a hearingpercept in the recipient, wherein at least two flanges extend away fromthe housing, the at least two flanges located at substantially oppositesides of the housing; and a magnet located outside the housing.
 2. Themedical implant of claim 1, wherein the at least two flanges haverespective top surfaces that extend in at least substantially in a planethat is parallel to a plane in which a top surface of the housingextends.
 3. The medical implant of claim 1, wherein with respect to aframe of reference looking downward on the implant normal to alongitudinal axis of the implant, an outboard most extension of theflanges extends further from the longitudinal axis than the outboardmost extension of the magnet.
 4. The medical implant of claim 1, whereinsaid at least two flanges are formed separately from at least a portionof the housing and mounted to the housing.
 5. The medical implant ofclaim 1, wherein said at least two flanges are removably mounted to thehousing.
 6. The medical implant of claim 5, wherein said at least twoflanges or the housing is provided with engagement means that areengageable with the housing or flanges, respectively.
 7. The medicalimplant of claim 1, further comprising: a receiver coil disposed in aportion of the medical implant outside the housing, the coil extendingabout the magnet, wherein with respect to a frame of reference lookingdownward on the implant normal to a longitudinal axis of the implant, anoutboard most extension of the flanges extends further from thelongitudinal axis than the outboard most extension of the coil.
 8. Themedical implant of claim 1, further comprising: a receiver coil disposedin a portion of the medical implant outside the housing, the coilextending about the magnet, wherein the at least two flanges have,respectively, one or more orifices passing therethrough that are adaptedto receive a tissue fixation device; and with respect to a frame ofreference looking downward on the implant normal to a longitudinal axis,the orifices are further from the longitudinal axis than the furthestextent of the magnet from the longitudinal axis.
 9. The medical implantof claim 1, wherein: with respect to a frame of reference lookingdownward on the implant normal to a longitudinal axis, the flanges arelocated at least about a center of the housing relative to positionalong the longitudinal axis.
 10. The medical implant of claim 1, furthercomprising: a receiver coil disposed in a portion of the medical implantoutside the housing, the coil extending about the magnet, wherein thereceiver coil and the housing collectively make up a receivercoil—housing assembly, and with respect to a frame of reference lookingdownward on the implant normal to a longitudinal axis, the flanges arelocated away from a center of the receiver coil—housing assembly. 11.The medical implant of claim 1, wherein said at least two flanges areformed separately from least a portion of the housing and mounted to theat least a portion of the housing.
 12. A medical implant configured tobe implanted in a recipient, the medical implant comprising: ahermetically sealed housing including at least two flanges attached toand extending outwardly from substantially opposite sides of thehousing, wherein the housing encases an electrical apparatus utilized toevoke a hearing percept in the recipient, and wherein respective flangesof the at least two flanges are securable to bone via respectiveapertures in the respective flanges of the at least two flanges; areceiver coil disposed in a portion of the medical implant outside thehousing; and a magnet, wherein the receiver coil extends about themagnet.
 13. The medical implant of claim 12, wherein at least one ofsaid at least two flanges has a top surface that extends in at leastsubstantially the same plane as that of a top surface of the housing.14. The medical implant of claim 12, wherein when a lower surface of thehousing is positioned within a well formed in an outer surface of askull bone and when the at least two flanges are secured to the bone, atleast a portion of the housing is disposed outside the well.
 15. Themedical implant of claim 12, wherein: the receiver coil is configured toreceive radio waves generated by a component located outside therecipient, the radio waves containing audio information; the medicalimplant is configured to provide a signal to the electrical apparatus inresponse to the received radio waves; and the electrical apparatus isconfigured to respond to the signal to evoke a hearing percept based onthe signal.
 16. The medical implant of claim 12, wherein said at leasttwo flanges are part of an apparatus that is a separate part from a partthat that makes up at least a portion of the housing.
 17. The housing ofclaim 12, wherein: the housing has a height that extends in a directionnormal to a direction of extension of at least one flange of the atleast two flanges away from the housing, wherein the housing isconfigured such that a bottom of the housing is located on an oppositeside from a top of the housing, the height being the distance betweenthe bottom and the top; and all flanges of the at least two flanges areentirely located substantially at a top of the housing.
 18. A medicalimplant configured to be implanted in a recipient, the implantcomprising: a hermetically sealed housing encasing an electricalapparatus utilized to evoke a hearing percept in the recipient, whereinthe medical implant includes two flanges extending outwardly fromsubstantially opposite first and second sides of the housing, there areonly two flanges extending from the housing, the medical implant isconfigured such that the bottom surfaces of the two flanges aresubstantially flush against the surface of the bone when a cavity isprovided for the housing in the bone and are securable to the bone viaan aperture in each flange, the two flanges are at a topmost portion ofthe housing, and the medical implant includes a receiver coil that ispositioned, with respect to the housing, at a location relative to athird side of the housing different from the first and second sides, amagnet being encircled by the receiver coil.
 19. The medical implant ofclaim 18, wherein the receiver coil is encased in a silicone bodyextending to a location, relative to a lateral axis of the medicalimplant, above the housing and above the topmost portion of the twoflanges.
 20. The medical implant of claim 18, wherein the receiver coilis part of an assembly that includes the magnet and silicone, whereinsilicone of the assembly extends to and over a top of the housing. 21.The medical implant of claim 18, wherein the receiver coil is part of anassembly that includes the magnet and silicone, wherein silicone of theassembly extends to and over only a portion of a top of the housing.